This invention is concerned with liquid ring vacuum pumps and compressors (hereinafter for the sake of brevity referred to simply as liquid ring pumps). The conventional liquid ring pump comprises a rotor having a plurality of longitudinally extending, generally radially disposed vanes which define working chambers or buckets. The rotor is disposed within an eccentric casing and a liquid seal, introduced into the casing, is caused by centrifugal force produced by the rotor, to form a ring following the casing. Since the casing is eccentric, the liquid of the ring alternately advances towards and recedes from the rotor axis to produce a pumping action within the buckets.
The casing may either define a single lobe, in which case its inner wall is generally circular and is centered on an axis offset from that about which the rotor turns so that there would be one pumping cycle per revolution of the rotor or the casing may define plural lobes (generally two lobes) in which case there are as many pumping cycles per revolution of the rotor as there are lobes. A typical single lobe pump is that described in U.S. Pat. No. 3,154,240 issued Oct. 27, 1964. A typical multiple lobe pump is shown in the U.S. Pat. No. 3,588,283 issued June 28, 1971.
Conventionally, pumps of the above types are constructed according to either a center port design or a side port design. In the center port design the port member is disposed within a cylindrically or conically shaped cavity within the rotor and has passages terminating in ports with which the open, radially inner ends of the rotor buckets are sequentially brought into register as the rotor turns. In the side port design, the port member is a radially disposed plate; the axial ends of the rotor buckets are at least partially open to and are brought sequentially into register with the openings on the port member corresponding to the intake and discharge regions of the pumping cycle. Pumps have also been constructed with a combination of these two arrangements with one of the inlet and outlet ports being in a radially disposed port member and the other being disposed in a central port member.
A feature common to all of these types of pumps is the disposition of a head leading to the ports at the longitudinal ends of the casing.
The parameters which largely determine the capacities of these pumps are the diameter and length of the rotor, which largely determine the rotor bucket volumes and the speed at which the rotor is turned. The operating tip speed of the rotor is limited by reason of performance and wear considerations to approximately 90 fps where the tip speed is defined as the tangential velocity of the rotor measured at its outer diameter. Generally, the 40 fps tip speed is a minimum below which the pump does not have a useful compression ratio. Above 50 fps the performance of the pump, reported as the horsepower required to pump a given volume of gas (reported as HP/CFM) generally deteriorates, i.e. the HP/CFM increases, in proportion to the square of the tip speed.
Because of the tip speed limitations, the RPM over which a pump may be operated efficiently is inversely proportional to its diameter. That is, a larger diameter pump must be run at a slower RPM to keep its tip speed in a commercially feasible range. However, since slower RPM motor drives are generally more expensive or involve expensive speed reduction equipment, it often is more attractive to gain capacity by increasing the pump length rather than by increasing its diameter.
It is also apparent that increased pump capacity gained by extending length as opposed to increasing diameter is desirable in those cases where the manufacturing operations have diametrical size limitations.
Prior to this invention, however, there has existed a restrictive limit to the length that a liquid ring pump of given rotor diameter could be constructed. This limit has been established by:
1. Aerodynamic considerations of the gas flow through the head and into and out of the rotor buckets; an excessive length to diameter ratio causes excessive pressure drop on the intake side, thus reducing volumetric efficiency.
2. Hydraulic considerations of the liquid ring; it is known that conventional liquid ring pumps are subject to hydraulic instability of the liquid ring when their length to diameter ratio is excessive. The instability manifests itself by the presence of noise and unusual wear, due to cavitation, the net effect of which is to make the product commercially unsuitable.
3. The strength of the shaft; it will be recognized that increasing the shaft diameter, to render it more resistant to bending, necessarily diminishes the capacity of the pump.
The net effect of these considerations is that the practical limit of liquid ring pump designs of the current state of the art is a length to diameter ratio of approximately 0.75 for a one sided entry rotor, i.e. a pump having a head at only one end of the rotor or 1.50 for a two sided entry rotor, i.e. a pump having a head at each axial end of the rotor.
In consequence of this, higher volume liquid ring pumps have necessarily been of limited length and have required large diameter rotors and the resulting lower speed drives. As pointed out, all of these factors lead to high manufacturing and motor drive costs.